Honeycomb filter

ABSTRACT

A honeycomb filter includes a honeycomb structure and a zeolite. The honeycomb structure has cell walls to define cells between the cell walls. The zeolite is supported on the cell walls. An amount of the zeolite is from about 80 g/L to about 150 g/L. The cells include a large volume cell and a small volume cell. A porosity of the cell walls is from about 55% to about 65%. A proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less. The pore volume A indicates a pore volume of pores having a pore diameter of one half or less of the average pore diameter. The pore volume B indicates a pore volume of pores having a pore diameter of twice or more the average pore diameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to PCT/JP2009/067672 filed on Oct. 9, 2009, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb filter.

2. Discussion of the Background

Particulate matter (hereinafter, also referred to as “PM”) such as soot is contained in exhaust gases discharged from internal combustion engines such as diesel engines, and has raised problems as contaminants harmful to the environment and the human body. Also, people have been worried about influence of toxic gas components such as CO, HC, and NOx contained in exhaust gases on the environment and the human body as well.

For the above reasons, exhaust gas purifying apparatuses for collecting PM or converting toxic gas component in exhaust gases have been used.

Honeycomb structures made of ceramics or the like are used for producing the exhaust gas purifying apparatuses. When exhaust gases are passed through the honeycomb structure, those gases can be purified (converted).

A honeycomb structure for collecting PM in exhaust gases in an exhaust gas purifying apparatus has a large number of cells each sealed at either end thereof and placed longitudinally in parallel with one another with a cell wall interposed therebetween. Therefore, exhaust gases flowing into one of the cells surely pass through the cell wall separating the cells and then flow out from other cells. Therefore, when a honeycomb structure of this kind is installed in an exhaust gas purifying apparatus, PM contained in exhaust gases are captured by the cell walls upon passing through the honeycomb structure. The cell walls of the honeycomb structure function as filters through which the exhaust gases are purified.

WO2005/002709 A1 discloses a honeycomb structure in which cells each sealed at an end on the gas outlet side are defined as cells having a large volume (hereinafter, also referred to as large volume cells) and cells each sealed at an end on the gas inlet side are defined as cells having a small volume (hereinafter, also referred to as small volume cells).

In the honeycomb structure of this kind disclosed in WO2005/002709 A1, a total area of apertures on the gas inlet side is relatively made larger than a total area of apertures on the gas outlet side so that the honeycomb structure, when used as a filter for purifying exhaust gases, can capture a large amount of PM.

WO2005/002709 A1 also discloses a honeycomb structure characterized by that pores have an average pore diameter from 5 to 30 μm; a proportion of the volume of fine pores having a pore diameter of twice or more the average pore diameter relative to the total volume of the pores is 30% or less.

According to WO2005/002709 A1, a honeycomb structure having the above features has a large effective filter area, and the period to a regeneration process can be prolonged.

On the other hand, as a honeycomb structure to be used for converting NOx in exhaust gases in an exhaust gas purifying apparatus, a honeycomb structure for converting NOx in which no end of the cells is sealed and a catalyst for converting NOx is supported on cell walls has been known.

The contents of WO2005/002709 A1 are incorporated herein by reference in their entirety.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a honeycomb filter includes a honeycomb structure and a zeolite. The honeycomb structure has cell walls extending along a longitudinal direction of the honeycomb structure to define cells between the cell walls. Each of the cells is sealed at either end of the cells. The zeolite is supported on the cell walls of the honeycomb structure. An amount of the zeolite supported on the cell walls is from about 80 g/L to about 150 g/L. The cells include a large volume cell and a small volume cell. A porosity of the cell walls of the honeycomb structure is from about 55% to about 65%. A proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less in a pore diameter distribution of the cell walls of the honeycomb structure. The pore volume A indicates a pore volume of pores having a pore diameter of one half or less of the average pore diameter. The pore volume B indicates a pore volume of pores having a pore diameter of twice or more the average pore diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a perspective view that schematically shows one example of a honeycomb structure forming the honeycomb filter according to the first embodiment of the present invention;

FIG. 2A is a perspective view that schematically shows one example of a honeycomb fired body forming the honeycomb structure shown in FIG. 1;

FIG. 2B is an A-A line cross-sectional view of the honeycomb fired body shown in FIG. 2A;

FIG. 3 is a graph that shows one example of the pore diameter distribution of the cell wall of the honeycomb structure according to an embodiment of the present invention;

FIG. 4 is a side view of the first end face schematically showing the cell structure of a base material 1 a;

FIG. 5 is a side view of the first end face schematically showing the cell structure of a base material 1 b;

FIG. 6 is a side view of the first end face schematically showing the cell structure of a base material 1 c;

FIG. 7A is a perspective view that schematically shows one example of the honeycomb structure forming the honeycomb filter according to the second embodiment of the present invention;

FIG. 7B is a B-B line cross-sectional view of the honeycomb structure shown in FIG. 7A; and

FIG. 8A, FIG. 8B, FIG. 8C, and FIG. 8D are each a side view that schematically shows one example of the first end face of the honeycomb fired body forming the aggregated honeycomb structure according to an embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

In conventional exhaust gas purifying apparatus, a honeycomb structure for capturing PM in exhaust gases and a honeycomb structure for converting NOx are different members respectively disposed in different metal containers, and thus they consist of a large volume in an exhaust gas line.

For this reason, decrease of the volume occupied by the exhaust gas purifying apparatus has been desired.

A urea SCR (Selective Catalytic Reduction) device has recently been proposed for converting NOx in exhaust gases.

In the urea SCR device, an aqueous urea solution is sprayed in an exhaust gas purifying apparatus provided with a honeycomb structure in which a catalyst such as zeolite is supported on cell walls. Ammonia is generated due to thermal decomposition of the urea, and the ammonia with the action of zeolite reduces NOx to N₂.

Accordingly, a urea SCR device can convert NOx.

The inventors of the present invention have tried to decrease the volume of a honeycomb filter and an exhaust gas purifying device in an exhaust gas line by allowing a single honeycomb filter to have both functions of NOx conversion and capturing of PM in a manner that zeolite as a catalyst is supported on cell walls of the honeycomb structure in which each of the cells is sealed at either end thereof, and the honeycomb structure is applied in the urea SCR device.

In this description, the honeycomb structure in which a catalyst such as zeolite is supported on the cell walls is referred to as honeycomb filter.

In the honeycomb filter applicable in the urea SCR device, it is considered necessary to increase the amount of zeolite to be supported on the cell walls of the honeycomb structure in order to increase the NOx conversion rate in the exhaust gas.

Further, in order to allow the cell walls of the honeycomb structure to support a large amount of zeolite, it is known to be necessary to increase the porosity of the honeycomb structure on which zeolite is to be supported.

The present inventors have manufactured a honeycomb filter in which a large amount of zeolite is supported on cell walls of a honeycomb structure by increasing the porosity of the conventional honeycomb structure described in WO2005/002709 A1.

In the case of the honeycomb filter manufactured by the foregoing method, however, the value of NOx conversion rate when it was used in a urea SCR device was not sufficient.

The present inventors investigated factors that have influences on the NOx conversion rate when the honeycomb filter is used in a urea SCR device.

As a result, the present inventors came to conceive that, in order to improve the NOx conversion rate, it is not only necessary to increase the porosity of the honeycomb structure by allowing the honeycomb structure to support a large amount of zeolite but to make zeolite sufficiently contact NOx.

The present inventors have thus found that, in a honeycomb structure having a large volume cell and a small volume cell, a large amount of zeolite tends to be supported on the cell walls and also NOx and zeolite are more likely to sufficiently contact with each other when the porosity of the cell walls of the honeycomb structure, the average pore diameter of the cell walls of the honeycomb structure, and the pore diameter distribution of the cell walls of the honeycomb structure are respectively controlled to be in predetermined ranges.

Namely, the honeycomb filter according to an embodiment of the present invention includes a honeycomb structure having a large number of cells each sealed at either end thereof and placed longitudinally in parallel with one another with a cell wall therebetween and a zeolite supported on the cell wall of the honeycomb structure, wherein an amount of the zeolite supported on the cell wall is from about 80 g/L to about 150 g/L, the large number of cells include a large volume cell and a small volume cell, a porosity of the cell wall of the honeycomb structure is from about 55% to about 65%, an average pore diameter of the cell wall of the honeycomb structure is from about 15 μm to about 25 μm, and a proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less in a pore diameter distribution of the cell wall of the honeycomb structure, the pore volume A indicating a pore volume of pores having a pore diameter of one half or less of the average pore diameter, the pore volume B indicating a pore volume of pores having a pore diameter of twice or more the average pore diameter.

In the honeycomb filter according to the embodiments of the present invention, the porosity of the cell wall of the honeycomb structure is from about 55% to about 65%. Therefore, the cell wall of the honeycomb structure tends to support a large amount of zeolite.

In the case where the porosity of the cell wall of the honeycomb structure is about 55% or more, when a large amount of zeolite is supported on the honeycomb structure, the pores of the cell wall are less likely to be clogged with zeolite, making it easier for exhaust gases to pass through the cell wall. As a result, the exhaust gases are more likely to be diffused, and the effect of zeolite is more likely to be sufficiently exerted.

On the other hand, in the case where the porosity of the cell wall of the honeycomb structure is about 65% or less, the heat capacity tends not to become too small. As a result, the temperature of the honeycomb filter is less likely to increase in a regeneration process for burning PM, and thus the catalyst is less likely to be deactivated.

Further, the porosity of the honeycomb structure about 65% or less tends not to reduce the strength of the honeycomb structure.

In the honeycomb filter according to the embodiments of the present invention, the amount of zeolite supported on the cell wall is from about 80 g/L to about 150 g/L.

When zeolite is supported on the cell wall in an amount of from about 80 g/L to about 150 g/L, the honeycomb filter used as a urea SCR device tends to sufficiently convert NOx in exhaust gas.

When the amount of zeolite supported is about 150 g/L or less, the pores in the cell wall of the honeycomb structure are less likely to be clogged if the pore diameter distribution is controlled, which may easily lead to sufficient contact between exhaust gas and zeolite.

Moreover, the average pore diameter of the cell wall of the honeycomb structure is from about 15 μm to about 25 μm in the honeycomb structure, and a proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less in a pore diameter distribution of the cell wall of the honeycomb structure, the pore volume A indicating a pore volume of pores having a pore diameter of one half or less of the average pore diameter, the pore volume B indicating a pore volume of pores having a pore diameter of twice or more the average pore diameter.

According to the above definition, supposing that the average particle diameter is “α” (μm), when the pore volume A is large, it is indicated that the volume of pores having a diameter belonging to the area of the pore diameter of smaller than one half the diameter of “α” (hereinafter also referred to as 0.5α) in the pore diameter distribution is large. On the other hand, when the pore volume B is large, it is indicated that the volume of pores having a diameter belonging to the area of the pore diameter of larger than twice the diameter of “α” (hereinafter also referred to as 2α) is large.

According to the above definition, a proportion of the sum of the pore volume A and the pore volume B relative to the total pore volume C is 20% or less. This shows that the volume of the pores having a pore diameter in a range of 0.5α to 2.0α exceeds 80% in the total pore volume C. In other words, a proportion of the pores having a pore diameter close to the average pore diameter is high, and thus the pore diameter distribution is uniform (narrow).

When the pore diameter distribution is uniform (narrow), a portion where exhaust gas hardly passes through and a portion where exhaust gas extremely easily passes through is less likely to appear, and thus the exhaust gases tend to uniformly pass through the cell wall. Therefore, the zeolite supported on the cell wall sufficiently tends to contact NOx in exhaust gas. As a result, a honeycomb filter having a high NOx conversion rate is more likely to be achieved.

In the honeycomb filter according to the embodiments of the present invention, a proportion of the pore volume A is about 10% or less of the total pore volume C, and a proportion of the pore volume B is about 10% or less of the total pore volume C.

In the cell wall of the honeycomb structure, a part including small-size pores is easily clogged with zeolite supported on the cell wall, hindering passage of exhaust gases. As a result, the zeolite supported on the part does not easily have a contact with the exhaust gases.

On the other hand, in a case where the cell wall of a honeycomb structure has a part including large-size pores, zeolite is not easily supported on the cell wall and exhaust gases tend to intensively pass through the part. As a result, zeolite does not easily have a contact with the exhaust gases.

In the honeycomb filter according to the embodiments of the present invention, the pore diameter distribution is controlled so as to set both the pore volume A and the pore volume B to be about 10% or less of the total pore volume C. As a result, exhaust gases tend to more uniformly pass through the cell wall and thus the zeolite supported on the cell wall tends to more sufficiently have a contact with NOx in the exhaust gases, making it easier to provide a honeycomb filter having an improved NOx conversion rate.

In the honeycomb filter according to the embodiments of the present invention, a cross section of the large volume cell perpendicular to the longitudinal direction has a substantially octagonal shape, and a cross section of the small volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape. The honeycomb filter including the cells having those cross sectional shapes is excellent in mechanical properties.

In the honeycomb filter according to the embodiments of the present invention, a cross section of the large volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape, and a cross section of the small volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape.

As the honeycomb filters according to the embodiments of the present invention have cells having the aforementioned cross sectional shapes, those honeycomb filters tend to be more preferably used for capturing PM in the exhaust gases and further for converting NOx in the exhaust gases.

In the honeycomb filter according to the embodiments of the present invention, the zeolite is at least one species selected from the group consisting of a β-type zeolite, a ZSM-5 type zeolite, and a SAPO.

As those zeolites are excellent in gas diffusion properties and resistance to water and heat, NOx in exhaust gases tends to be preferably converted.

In the honeycomb filter according to claim 6, the zeolite is ion-exchanged with a copper ion and/or an iron ion.

In the honeycomb filter according to the embodiments of the present invention, the honeycomb structure includes a plurality of honeycomb fired bodies combined with one another with an adhesive layer interposed therebetween.

First Embodiment

The following description will discuss the first embodiment of the present invention which is one embodiment of the honeycomb filter.

The honeycomb filter of the present embodiment is provided by allowing the cell wall of the honeycomb structure to support zeolite.

As mentioned earlier, in the present description, a honeycomb structure in which zeolite is not supported on the cell wall is referred to as “honeycomb structure” and a honeycomb structure in which zeolite is supported on the cell wall is referred to as “honeycomb filter” to differentiate them from each other.

The honeycomb structure includes the honeycomb fired body, and the cell and the cell wall of the honeycomb structure also mean the cell and the cell wall of the honeycomb fired body.

FIG. 1 is a perspective view that schematically shows one example of a honeycomb structure forming the honeycomb filter according to the first embodiment of the present invention.

FIG. 2A is a perspective view that schematically shows one example of a honeycomb fired body forming the honeycomb structure shown in FIG. 1, and FIG. 2B is an A-A line cross-sectional view of the honeycomb fired body shown in FIG. 2A.

A honeycomb structure 10 shown in FIG. 1 is provided by bonding a plurality of honeycomb fired bodies 20 each made of a porous ceramic with one another by interposing an adhesive layer 11 therebetween to construct a ceramic block 13, and then forming a coat layer 12 for preventing leak of exhaust gases around the ceramic block 13. The coat layer 12 may be formed when necessary.

A honeycomb structure provided by bonding a plurality of honeycomb fired bodies as mentioned earlier is referred also as an aggregated honeycomb structure.

The main component of constituent materials of the aggregated honeycomb structure is desirably silicon carbide or silicon carbide containing silicon.

The honeycomb structure 10 includes a plurality of cells placed in parallel with one another with a cell wall interposed therebetween in a longitudinal direction (direction of the double-sided arrow “a” in FIG. 1), a first end face 14 and a second end face 15. The following description will discuss positional relation among the first end face 14, the second end face 15 and the plurality of the cells.

In the honeycomb fired body 20 shown in FIG. 2A and FIG. 2B, the plurality of cells include large volume cells 21 a each having a relatively larger cross-sectional area perpendicular to the longitudinal direction thereof (direction of the double-sided arrow “b” in FIG. 2A) than small volume cells 21 b, and the small volume cells 21 b each having a relatively smaller cross-sectional area perpendicular to the longitudinal direction thereof than the large volume cells 21 a, and the large volume cells 21 a and the small volume cells 21 b are alternately disposed one by one.

The large volume cell 21 a has a substantially octagonal shape in the cross section perpendicular to the longitudinal direction thereof. The small volume cell 21 b has a substantially quadrangular shape in the cross section perpendicular to the longitudinal direction thereof.

The honeycomb fired body 20 has a first end face 24 and a second end face 25.

The large volume cell 21 a is open at an end portion on the first end face 24 side of the honeycomb fired body 20 and is sealed with a sealing material 22 a at an end portion on the second end face 25 side of the honeycomb fired body 20. On the other hand, the small volume cell 21 b is open at an end portion on the second end face 25 side of the honeycomb fired body 20 and is sealed with a sealing material 22 b at an end portion on the first end face 24 side of the honeycomb fired body 20.

A cell wall 23 interposed between the large volume cell 21 a and the small volume cell 21 b functions as a filter.

Namely, exhaust gases “G” (in FIG. 2B, “G” indicates exhaust gases and the arrow indicates a flowing direction of exhaust gases) introduced to the large volume cell 21 a surely pass through the cell wall 23 interposed between the large volume cell 21 a and the small volume cell 21 b and then flow out from the small volume cell 21 b.

The honeycomb structure 10 includes a plurality of the honeycomb fired bodies 20 which are arranged and bonded with one another in a manner that the first end face 24 of each of the honeycomb fired bodies 20 forms the first end face 14 of the honeycomb structure 10, and the second end face 25 of each of the honeycomb fired bodies 20 forms the second end face 15 of the honeycomb structure body 10.

Accordingly, in the honeycomb structure 10, the large volume cells 21 a are open at the end portions on the first end face 14 of the honeycomb structure and are sealed at the end portions on the second end face 15. On the other hand, the small volume cells 21 b are open at the end portions on the second end face 15 of the honeycomb structure 10 and are sealed at the end portions on the first end face 14.

The cross-sectional shape of each of the large volume cells and that of each of the small volume cells perpendicular to the longitudinal direction thereof may be other than the shape shown in FIG. 2A and FIG. 2B. The cross-sectional shape of the large volume cell perpendicular to the longitudinal direction may be a substantially quadrangular shape, and the cross-sectional shape of the small volume cell perpendicular to the longitudinal direction thereof may be a substantially quadrangular shape.

The area ratio of the cross-sectional area of the large volume cell perpendicular to the longitudinal direction relative to the cross-sectional area of the small volume cell perpendicular to the longitudinal direction (cross-sectional area of the large volume cell perpendicular to the longitudinal direction/cross-sectional area of the small volume cell perpendicular to the longitudinal direction) is preferably from about 1.4 to about 2.4.

When the area ratio is about 1.4 or more, the effect to be obtained by including the large volume cells and the small volume cells may be sufficiently obtained easily. When the area ratio is about 2.4 or less, a proportion of the cell wall interposed between the large volume cells is less likely to become high, and thus the amount of zeolite to be supported on the cell wall interposed between the large volume cell and the small volume cell is more likely to be increased. As exhaust gases easily pass through the cell wall interposed between the large volume cell and the small volume cell, the zeolite supported on the cell wall tends to contribute to NOx conversion. As a result, NOx conversion rate upon using the honeycomb filter for a urea SCR device is less likely to be decreased.

The porosity of the cell wall of the honeycomb structure is from about 55% to about 65%.

In the present description, the porosity of the cell wall of the honeycomb structure can be measured through a conventionally known method such as a mercury porosimetry method, Archimedes method, a weighing method, and a measuring method using a scanning electronic microscope (SEM).

In the present embodiment, the porosity of the cell wall of the honeycomb structure refers to the porosity of the cell wall of the honeycomb fired body which forms the honeycomb structure.

The average pore diameter of the cell wall of the honeycomb structure is from about 15 μm to about 25 μm.

In a pore diameter distribution of the cell wall of the honeycomb structure, a proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less in a pore diameter distribution of the cell wall of the honeycomb structure, the pore volume A indicating a pore volume of pores having a pore diameter of one half or less of the average pore diameter, the pore volume B indicating a pore volume of pores having a pore diameter of twice or more the average pore diameter.

More preferably, a proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 9% or less.

Preferably, the pore volume A is about 10% or less of the total pore volume C, and the pore volume B is about 10% or less of the total pore volume C.

In the present embodiment, the pore diameter distribution of the cell wall of the honeycomb structure refers to the pore diameter distribution of the cell wall of the honeycomb fired body which forms the honeycomb structure.

The average pore diameter and the pore diameter distribution of the cell wall of the honeycomb structure can be measured by mercury porosimetry method (according to JIS R 1655:2003).

FIG. 3 is a graph that shows one example of the pore diameter distribution (relation between pore diameter (μm) and accumulated pore volume (%)) of the cell wall of the honeycomb structure according to the embodiments of the present invention (before zeolite is supported).

In the graph of FIG. 3, the bold line shows the pore diameter distribution, with the intensity corresponding to the number of pores having diameters indicated on the horizontal axis. The thin line shows the accumulated pore volume, with the intensity of the accumulated pore volume corresponding to the pore volume (%) of pores having pore diameters larger than the pore diameters indicated on the horizontal axis in which the total pore volume C is set to 100%.

The contents of JIS R 1655 (2003) are incorporated herein by reference in their entirety.

In the graph of FIG. 3, the average pore diameter (α) of the cell wall of the honeycomb structure is 20 μm. The pore diameter of one half of the average pore diameter α is 10 μm (0.5α), and the pore diameter twice the average pore diameter α is 40 μm (2α). The graph of FIG. 3 shows the positions of the average pore diameter α, the diameter 0.5α, and the diameter 2.0α.

The pore volume A of pores having a pore diameter of 0.5α or less is read to be 4.7% based on the value 95.3% of the accumulated pore volume (%) at a pore diameter of 10 μm in the graph of FIG. 3.

The pore volume B of pores having a pore diameter of 2α or more is read to be 6.2% based on the value 6.2% of the accumulated pore volume (%) at a pore diameter of 40 μm in the graph of FIG. 3.

Calculation of a sum of the pore volume A and the pore volume B obtained above gives the value of 10.9%.

Further, zeolite is supported on the cell wall of the honeycomb structure.

In the present description, zeolite includes not only aluminosilicate but analogues of zeolite such as aluminophosphate and aluminogermanate as well.

Examples of zeolite to be supported on the cell wall of the honeycomb structure include β-type zeolite, Y-type zeolite, ferrierite, ZSM-5 type zeolite, mordenite, faujasite, A-type zeolite, L-type zeolite, SAPO (Silicoaluminophosphate), MeAPO (Metalaluminophosphate), and the like. Each of these may be used alone or two or more kinds of these may be used in combination.

β-type zeolite, ZSM-5 type zeolite, or SAPO is preferable among the above zeolites. Preferable among SAPO is SAPO-5, SAPO-11, or SAPO-34, and SAPO-34 is more preferable. Preferable among MeAPO is MeAPO-34.

The amount of zeolite (hereinafter, also referred to as zeolite support amount) supported on the cell wall of the honeycomb structure is preferably from about 80 g/L to about 150 g/L.

The amount of zeolite is preferably from about 120 g/L to about 150 g/L.

In the present description, the amount of zeolite supported on the cell wall of the honeycomb structure refers to the weight of zeolite per one liter apparent volume of the honeycomb structure.

The apparent volume of the honeycomb structure includes a volume of adhesive layers and coat layers.

The zeolite is preferably ion-exchanged with metal ions. Examples of the metal ions include a copper ion, an iron ion, a nickel ion, a zinc ion, a manganese ion, a cobalt ion, a silver ion, a vanadium ion, and the like. Each of these may be used alone or two or more kinds of these may be used in combination.

Preferable among the metal ions is copper ion or iron ion.

The following description will discuss one example of the method for manufacturing a honeycomb filter according to the present embodiment. Here, explanation will be made on a method for manufacturing a honeycomb filter in which zeolite is supported on the cell wall of a honeycomb structure formed of the honeycomb fired body shown in FIG. 2A and FIG. 2B.

First, a wet mixture for manufacturing a molded body is prepared by mixing silicon carbide powders having different average particle diameters as a ceramic material, an organic binder, a pore-forming agent, a liquid plasticizer, a liquid lubricant, and water.

In the method for manufacturing the honeycomb structure according to the present embodiment, a technique of controlling the particle diameter of hollow acrylic particles to be used as a pore-forming agent may be employed so as to control the pore diameter distribution of the cell wall of the honeycomb structure in a predetermined range.

Specifically, the hollow acrylic particles to be used as a pore-forming agent are sieved to sufficiently remove coarse or micro particles so that the hollow acrylic particles having a uniform (narrow) particle diameter distribution are prepared. The resulting hollow acrylic particles are used for manufacturing the honeycomb structure.

The range of the particle diameter of the particles to be removed in sieving of the hollow acrylic particles may be optionally set depending on the range in which the pore diameter distribution needs to be adjusted. For example, in the particle diameter distribution of the hollow acrylic particles, 90% of the particles (volume) are included in a range of ±5 μm of the average particle diameter. Also, for example, the particle diameter distribution of the hollow acrylic particles of D5-D95 may be included in a range of ±5 μm of the average particle diameter.

Moreover, in the case where a pore-forming agent other than the hollow acrylic particles is used, by similarly preparing particles having a uniform particle diameter distribution, the pore diameter distribution of the cell wall can be adjusted in a predetermined range.

One of other methods to control the pore diameter distribution of the cell wall in a predetermined range is a method to allow the ceramic particles to have a uniform particle diameter. Similarly as in the aforementioned method of allowing the hollow acrylic particles to have a uniform particle diameter, ceramic particles having a uniform particle diameter can be obtained by sieving ceramic particles to remove the ceramic particles having predetermined ranges of diameter. Then, by using the ceramic particles having a uniform particle diameter, the pore diameter distribution of the cell wall can be adjusted in a predetermined range.

Next, the wet mixture is introduced into an extrusion molding apparatus and then extrusion-molded so as to manufacture a honeycomb molded body having a predetermined shape.

The honeycomb molded body is manufactured by using a die which can form a honeycomb molded body in which a large volume cell having a substantially octagonal shape and a large area in the cross section perpendicular to the longitudinal direction and a small volume cell having a substantially quadrangular shape and a small area in the cross section perpendicular to the longitudinal direction are alternately disposed, and the cell density, the thickness of the cell wall, and the ratio of the cross-sectional area of the large volume cell perpendicular to the longitudinal direction relative to the cross-sectional area of the small volume cell perpendicular to the longitudinal direction in the honeycomb molded body are respectively adjusted in predetermined ranges.

Next, cutting of both ends of the honeycomb molded body with a cutting apparatus is performed to cut the honeycomb molded body into a predetermined length. The cut honeycomb molded body is dried with a drying apparatus.

Then, a predetermined amount of a sealing material paste which is to become a sealing material is filled in one end of each of the large volume cells and one end of each of the small volume cells of the dried honeycomb molded body to seal the cells. Through the aforementioned process, a cell-sealed honeycomb molded body is manufactured.

As the sealing material paste, the wet mixture may be used.

Thereafter, degreasing is carried out to heat organic matters in the cell-sealed honeycomb molded body so that a honeycomb degreased body is manufactured. Shape of the honeycomb degreased body is almost the same as the shape of the honeycomb fired body shown in FIG. 2A and FIG. 2B.

The honeycomb degreased body is transported to a firing furnace and then fired at a temperature of from about 2000° C. to about 2300° C. under argon atmosphere, and thereby a honeycomb fired body having a shape shown in FIG. 2A and FIG. 2B is manufactured.

It is to be noted that the pore diameter distribution may also be changed by controlling the firing conditions (firing temperature, firing time).

Next, binding is performed by applying an adhesive paste between the honeycomb fired bodies to form an adhesive paste layer and then drying and solidifying the adhesive paste layer to form an adhesive layer. Accordingly, a ceramic block in which a plurality of the honeycomb fired bodies are bonded with one another with an adhesive layer interposed therebetween is manufactured.

The adhesive paste to be preferably used is an adhesive paste including inorganic fibers and/or whiskers, an inorganic binder, and an organic binder.

In the binding, the plurality of the honeycomb fired bodies are arranged so that the first end faces of respective honeycomb fired bodies are disposed on the same side, and then the honeycomb fired bodies are bonded with one another.

Then, periphery cutting is carried out by cutting the periphery of the ceramic block with a diamond cutter so as to provide a substantially round pillar-shaped ceramic block.

Next, coating layer forming is performed by applying a coating material paste on the peripheral surface of the substantially round pillar-shaped ceramic block, and then drying and solidifying the coating material paste to form a coating layer.

As the sealing material paste, a paste that is the same as the adhesive paste may be used.

Through the foregoing process, a honeycomb structure can be manufactured.

Thereafter, a zeolite such as β-type zeolite ion-exchanged with an iron ion is supported on the cell wall of the honeycomb structure.

Zeolite is supported on the cell wall of the honeycomb structure by such a method, for example, that the honeycomb structure is dipped in a slurry containing zeolite, pulled up, and then heated.

The amount of zeolite to be supported may be controlled by a method of repeating the dipping of the honeycomb structure in a slurry and the heating, a method of changing the concentration of the slurry, or the like.

Through the foregoing process, it is possible to manufacture a honeycomb filter in which zeolite is supported on the cell wall of the honeycomb structure.

The following description will discuss effects of the honeycomb filter of the present embodiment.

(1) In the honeycomb filter according to the present embodiment, the plurality of cells in the honeycomb structure include large volume cells and small volume cells. Therefore, it may become easier to capture a large amount of PM in exhaust gases.

(2) In the honeycomb filter according to the present embodiment, the porosity of the cell wall of the honeycomb structure is from about 55% to about 65%. Therefore, a large amount of zeolite is more likely to be supported on the cell wall of the honeycomb structure.

(3) In the honeycomb filter according to the present embodiment, the average pore diameter of the cell wall of the honeycomb structure is from about 15 μm to about 25 μm, and a proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less in a pore diameter distribution of the cell wall of the honeycomb structure, the pore volume A indicating a pore volume of pores having a pore diameter of one half or less of the average pore diameter, the pore volume B indicating a pore volume of pores having a pore diameter of twice or more the average pore diameter.

In other words, the pore diameter distribution of the cell wall is uniform (narrow), and the cell wall tends not to have a part where exhaust gases hardly pass through or where exhaust gases tend to intensively pass through. Therefore, the exhaust gases tend to uniformly pass through the cell wall. Therefore, the zeolite supported on the cell wall tends to sufficiently contact NOx in exhaust gases, and as a result a honeycomb filter having a high NOx conversion rate is more likely to be obtained.

(4) In the honeycomb filter according to the present embodiment, the pore volume A is about 10% or less of the total pore volume C, and the pore volume B is about 10% or less of the total pore volume C. Thus, exhaust gases tend to more uniformly pass through the cell wall. Therefore, the zeolite supported on the cell wall tends to more sufficiently contact NOx in exhaust gases, and as a result a honeycomb filter having a higher NOx conversion rate is more likely to be obtained.

(5) In the honeycomb filter according to the present embodiment, the amount of the zeolite supported on the cell wall is from about 80 g/L to about 150 g/L. Therefore, use of the honeycomb filter of the embodiment of the present invention as a urea SCR device makes it easier to sufficiently convert NOx in exhaust gases.

EXAMPLES

The following description will discuss examples which specifically disclose the first embodiment of the present invention. The present invention is not limited only to those examples.

First, base materials 1 to 7 having different pore diameter distributions from one another were prepared.

(Manufacturing of Base Material 1)

Three kinds of the base materials 1 a, 1 b, and 1 c having a different cell cross-sectional shape from one another were manufactured as the base material 1.

(Manufacturing of Base Material 1 a)

An amount of 46.6% by weight of coarse powder of silicon carbide having an average particle diameter of 20 μm and an amount of 20.0% by weight of fine powder of silicon carbide having an average particle diameter of 0.5 μm were mixed, and to the resulting mixture were added and kneaded 6.8% by weight of hollow acrylic particles having an average particle diameter of 21 μm as a pore-forming agent, 3.8% by weight of an organic binder (methylcellulose), 3.5% by weight of a lubricant (UNILUB, made by NOF Corporation), 1.6% by weight of a plasticizer (glycerin), and 17.7% by weight of water to prepare a wet mixture. The wet mixture was then extrusion-molded with a die to manufacture a raw honeycomb molded body having virtually the same shape as the shape shown in FIG. 2A and FIG. 2B with no cells being sealed.

The hollow acrylic particles used were hollow acrylic particles in which the particle diameter was adjusted by removing coarse particles having a particle diameter of more than 26 μm and micro particles having a particle diameter of less than 16 μm by sieving so that a proportion of a sum of volume of the micro particles and volume of the coarse particles is 10% or less of the total volume of the particles in the particle diameter distribution.

Next, the raw honeycomb molded body was dried by using a microwave drying apparatus to obtain a dried body of the honeycomb molded body. Thereafter, a paste having the same composition as that of the raw molded body was filled into predetermined cells, and then again dried by a drying apparatus.

The dried honeycomb molded body was degreased at 400° C., and then fired at 2200° C. under normal pressure argon atmosphere for 3 hours so as to manufacture a honeycomb fired body made of a silicon carbide sintered body with a porosity of 60%, an average pore diameter of 20 μm, a size of 34.3 mm×34.3 mm×150 mm, the number of cells (cell density) of 350 pcs/inch² and a thickness of the cell wall of 0.28 mm (11 mil). The honeycomb fired body manufactured through the above process was given as a base material 1 a.

FIG. 4 is a side view of the first end face schematically showing the cell structure of the base material 1 a.

In the base material 1 a, as shown in FIG. 4, a cross sectional shape of a large volume cell 51 a of the honeycomb fired body 50 is octagonal, and the length shown by the double-sided arrow “X” is 1.21 mm. A cross sectional shape of a small volume cell 51 b is a quadrangle (substantially square), and the length of one side thereof (shown by the double-sided arrow “Y” in FIG. 4) is 0.97 mm. The thickness (shown by the double-sided arrow “Z” in FIG. 4) of a cell wall 53 between the large volume cell 51 a and the small volume cell 51 b is 0.28 mm.

An area of a cross section of the large volume cell perpendicular to the longitudinal direction is 1.40 mm², and an area of a cross section of the small volume cell perpendicular to the longitudinal direction is 0.94 mm². Therefore, an area ratio of the area of the cross section of the large volume cell perpendicular to the longitudinal direction relative to the area of the cross section of the small volume cell perpendicular to the longitudinal direction is 1.5.

(Manufacturing of Base Material 1 b and Base Material 1 c)

FIG. 5 is a side view of the first end face schematically showing the cell structure of the base material 1 b. FIG. 6 is a side view of the first end face schematically showing the cell structure of the base material 1 c.

The base material 1 b and the base material 1 c which were honeycomb fired bodies respectively having the cell structures with the shapes shown in FIG. 5 and FIG. 6 were manufactured according to the aforementioned manufacturing of the base material 1 a while changing the shapes of the dies used in the extrusion molding.

In the base material 1 b, as shown in FIG. 5, the cross sectional shape of a large volume cell 61 a of the honeycomb fired body 60 is octagonal, and the length shown by the double-sided arrow “X” is 1.37 mm. The cross sectional shape of a small volume cell 61 b is a quadrangle (substantially square), and the length of one side thereof (shown by the double-sided arrow “Y” in FIG. 5) is 0.87 mm. The thickness (shown by the double-sided arrow “Z” in FIG. 5) of a cell wall 63 between the large volume cell 61 a and the small volume cell 61 b is 0.28 mm.

An area of a cross section of the large volume cell perpendicular to the longitudinal direction is 1.81 mm², and an area of a cross section of the small volume cell perpendicular to the longitudinal direction is 0.76 mm². Therefore, an area ratio of the area of the cross section of the large volume cell perpendicular to the longitudinal direction relative to the area of the cross section of the small volume cell perpendicular to the longitudinal direction is 2.4.

In the base material 1 c, as shown in FIG. 6, the cross sectional shape of a large volume cell 71 a of the honeycomb fired body 70 is a quadrangle (substantially square), and the length shown by the double-sided arrow “X” is 1.18 mm. The cross sectional shape of a small volume cell 71 b is a quadrangle (substantially square), and the length of one side thereof (shown by the double-sided arrow “Y” in FIG. 6) is 0.97 mm. The thickness (shown by the double-sided arrow “Z” in FIG. 6) of a cell wall 73 between the large volume cell 71 a and the small volume cell 71 b is 0.28 mm.

An area of a cross section of the large volume cell perpendicular to the longitudinal direction is 1.39 mm², and an area of a cross section of the small volume cell perpendicular to the longitudinal direction is 0.94 mm². Therefore, an area ratio of the area of the cross section of the large volume cell perpendicular to the longitudinal direction relative to the area of the cross section of the small volume cell perpendicular to the longitudinal direction is 1.5.

The porosity, average pore diameter, and pore diameter distribution of the base material 1 b and the base material 1 c are the same as those of the base material 1 a.

The base material 1 a, the base material 1 b, and the base material 1 c will be collectively referred to as base material 1 herein below. Tables 1-1 and 1-2 show the composition of the wet mixture and firing conditions used for manufacturing the base material 1.

(Manufacturing of Base Materials 2 to 5)

Base materials 2 to 5 were manufactured by manufacturing honeycomb fired bodies in the same manner as the manufacturing of the base material 1 a, except that the composition and the firing conditions of the wet mixture were changed as shown in Tables 1-1 and 1-2. The shapes of the base materials 2 to 5 are the same as that of the base material 1 a.

(Manufacturing of Base Materials 6 and 7)

Base materials 6 and 7 were manufactured by manufacturing honeycomb fired bodies in the same manner as the manufacturing of the base material 1 a, except that hollow acrylic particles whose particle diameters were not adjusted with a sieve were used in the composition of the wet mixture, and further the composition and the firing conditions of the wet mixture were changed as described in Tables 1-1 and 1-2. Shapes of the base materials 6 and 7 are the same as that of the base material 1 a.

Tables 1-1 and 1-2 collectively show the composition of the wet mixture and firing conditions used for manufacturing the base materials 1 to 7.

TABLE 1-1 SiC coarce powder SiC fine powder Methyl Average Average cellulose pore diam- Blending pore diam- Blending Blending eter (μm) ratio (%) eter (μm) ratio (%) ratio (%) Base mate- 20 46.6 0.5 20.0 3.8 rial 1a, 1b, 1c Base mate- 20 47.5 0.5 20.3 3.9 rial 2 Base mate- 25 45.7 0.5 19.7 3.7 rial 3 Base mate- 20 48.3 0.5 20.8 3.9 rial 4 Base mate- 25 44.9 0.5 19.2 3.7 rial 5 Base mate- 25 46.6 0.5 20.0 3.8 rial 6 Base mate- 25 46.6 0.5 20.0 3.8 rial 7

Pore forming agent (Note 1) Lubricant Plasticizer Water Firing temperature Average pore Blending Blending Blending Blending Temperature Time diameter (μm) ratio (%) ratio (%) ratio (%) ratio (%) (° C.) (Hr) Base material 21 6.8 3.5 1.6 17.7 2200 3 1a, 1b, 1c Base material 21 5.1 3.6 1.6 18.0 2150 3 2 Base material 21 8.5 3.4 1.6 17.4 2200 3 3 Base material 21 3.4 3.6 1.7 18.3 2200 3 4 Base material 21 10.2 3.4 1.5 17.1 2150 3 5 Base material 40 6.8 3.5 1.6 17.7 2150 3 6 Base material 30 6.8 3.5 1.6 17.7 2150 3 7 (Note 1) In base materials 1 to 5, hollow acrylic powder whose particle diameter is controlled by sieving in a predetermined range is used.

(Measurement of Porosity, Average Pore Diameter, and Pore Diameter Distribution)

With regard to the base materials 1 to 7, the porosity was measured by a weighing method, and the average particle diameter and the pore diameter distribution (0.2 to 500 μm) were measured by a mercury porosimetry method (in accordance with JIS R 1655:2003).

As a specific measuring procedure by a mercury porosimetry method, each base material was cut into cubes having a size of about 0.8 cm, and these were washed by using ultrasonic-wave with ion exchanged water, and sufficiently dried to give samples. Next, the pore diameters of these samples were measured by using a Micromeritics automatic porosimeter, AutoPore III9405, product of Shimadzu Corporation. In the measuring, the measuring range was set from 0.2 to 500 μm, and in the range from 100 to 500 μm, the measurements were carried out for every pressure unit of 0.1 psia, and in the range from 0.2 to 100 μm, the measurements were carried out for every pressure unit of 0.25 psia. Thus, the pore diameter distribution and the total pore volume C are calculated.

The average pore diameter α was calculated as the value obtained from (4×S (accumulated pore area)/V (accumulated pore volume)).

Moreover, a pore diameter of one half of the average pore diameter α (0.5α) and a pore diameter of twice the average pore diameter (2α) were calculated. Then, a pore volume A of pores having a pore diameter of 0.5α or less and a pore volume B of pores having a pore diameter of 2α or more were calculated.

Furthermore, a proportion (%) of the pore volume A relative to the total pore volume C and a proportion (%) of the pore volume B relative to the total pore volume C were calculated, and the sum (A+B) of the above two proportions was further obtained. Tables 2-1 and 2-2 show the results.

(Manufacturing of Honeycomb Structure)

Honeycomb structures 1 to 7 were manufactured by using the base materials 1 to 7, respectively, as honeycomb fired bodies.

The honeycomb structures manufactured by using the base materials 1 a, 1 b, 1 c are referred to as the honeycomb structures 1 a, 1 b, 1 c, respectively. Also, the honeycomb structures 1 a, 1 b, 1 c are collectively referred to as a honeycomb structure 1.

Additionally, the honeycomb structures manufactured by using the base materials 2 to 7 as the honeycomb fired bodies were referred to as honeycomb structures 2 to 7, respectively.

The honeycomb structures 1 to 7 were manufactured according to the following procedure.

An adhesive paste was applied between the honeycomb fired bodies to form adhesive paste layers, and the adhesive paste layers were dried and solidified to form adhesive layers. Accordingly, a ceramic block having a substantially quadrangular pillar shape in which sixteen pieces of the honeycomb fired bodies were bonded with one another with the adhesive layer interposed therebetween was manufactured.

In this procedure, a plurality of the honeycomb fired bodies were bonded in a manner that the first end faces of the respective honeycomb fired bodies were arranged on the same side.

As the adhesive paste, an adhesive paste containing 30% by weight of alumina fiber having an average fiber length of 20 μm, 21% by weight of silicon carbide powder having an average particle diameter of 0.6 μm, 15% by weight of silica sol (solid content 30% by weight), 5.6% by weight of carboxymethyl cellulose, and 28.4% by weight of water was used.

Thereafter, periphery cutting was performed on the quadrangular pillar-shaped ceramic block so that a round pillar-shaped ceramic block having a diameter of 142 mm was manufactured.

Next, a coating material paste was applied on the periphery of the round pillar-shaped ceramic block, and the coating material paste was dried and solidified at a temperature of 120° C. so that a coat layer was formed on the periphery of the ceramic block.

As the coating material paste, the same paste as the aforementioned adhesive paste was used.

Through the above procedure, a round pillar-shaped honeycomb structure having a size of 143.8 mm in diameter×150 mm in length was manufactured.

Next, honeycomb filters were manufactured in the following Examples, Reference Examples, and Comparative Examples by allowing the honeycomb structures 1 to 7 manufactured by using the base materials 1 to 7 to support zeolite.

Example 1

A sufficient amount of water is mixed with β-type zeolite powder (average particle diameter: 2 μm) ion-exchanged with an iron ion and then stirred to prepare a zeolite slurry. The honeycomb structure 1 a was dipped in the zeolite slurry with one end side facing down for one minute. Thereafter, the resulting honeycomb structure 1 a was dried at 110° C. for one hour, and further fired at 700° C. for one hour so that a zeolite supporting layer was formed.

The procedure of dipping in the zeolite slurry, drying and firing was repeated so that the amount of the formed zeolite supporting layer reached 120 g per one liter of the apparent volume of the honeycomb structure.

Through the above procedure, a honeycomb filter having a zeolite supporting amount of 120 g/L (apparent volume) was manufactured.

Examples 2 to 7, Reference Example 1, Comparative Examples 1 to 5

Honeycomb filters were manufactured in Examples 2 to 7, Reference Example 1, and Comparative Examples 1 to 5, by allowing the honeycomb structures manufactured by respectively using the base materials shown in Tables 2-1 and 2-2 to support zeolite in respective amounts shown in Tables 2-1 and 2-2.

The amounts of zeolite to be supported were controlled by changing the number of times to repeat the dipping into zeolite slurry, drying or firing.

(Measurement of Nox Conversion Rate)

NOx conversion rate was measured on the honeycomb filters manufactured in Examples, Reference Example, and Comparative Examples.

For measurement of NOx conversion rate, each of the honeycomb filters manufactured in Examples, Reference Example, and Comparative Examples was cut by a diamond cutter to produce a single honeycomb fired body (34.3 mm×34.3 mm×150 mm). The cut-out honeycomb fired body was further cut shortened to prepare a short-length body in a size of 34.3 mm×34.3 mm×40 mm.

In the same manner as the aforementioned sealing and degreasing, cells of the short-length body were sealed so that one end of each of the cells of the short-length body was sealed with the adhesive paste. The short-length body with the sealed cells was degreased at 400° C. so that a sample for measuring NOx conversion rate was prepared.

The NOx conversion rate was measured with a NOx conversion rate-measuring apparatus (Catalyst test system SIGU-2000, product of HORIBA, Ltd.).

The NOx conversion rate-measuring apparatus includes a gas generator and a reactor. A simulated exhaust gas generated by the gas generator was passed through the reactor in which the sample for evaluation was placed.

Composition of the simulated exhaust gas includes NO:175 ppm, NO₂:175 ppm, NH₃:350 ppm, O₂:14%, CO₂:5%, H₂O:10%, and N₂:balance. This composition was obtained by controlling each the amount of the gas flow rate with a flow rate controller.

The temperature of the reactor was fixed at 200° C. As a condition to allow zeolite to contact the simulated exhaust gas, the space velocity (SV) was set at 70000 hr⁻.

The NOx concentration N₀ before the simulated exhaust gas passed through the sample for evaluation and the NOx concentration N₁ after the simulated exhaust gas passed through the sample for evaluation were measured, and then the NOx conversion rate was calculated in accordance with the following equation.

NOx conversion rate(%)=[(N₀-N₁)/N₀]×100

Tables 2-1 and 2-2 show the measurement results of the NOx conversion rate.

Tables 2-1 and 2-2 collectively show types of the base materials, shapes of the cells, porosity, average pore diameter α, proportion of the pore volume (pore volume A, pore volume B, total (A+B)) relative to the total pore volume C, zeolite supporting amount (apparent volume), and NOx conversion rate according to Examples and Reference Example, and Comparative Examples.

TABLE 2-1 Average Base Shape Porosity pore diameter material  (Figure) (%) α(μm) Example 1  1a FIG. 4 60 20 Example 2  1b FIG. 5 60 20 Example 3  1c FIG. 6 60 20 Example 4 2 FIG. 4 55 18 Example 5 3 FIG. 4 65 23 Example 6 1 FIG. 4 60 20 Example 7 1 FIG. 4 60 20 Reference 7 FIG. 4 60 20 Example 1 Comparative 4 FIG. 4 50 20 Example 1 Comparative 5 FIG. 4 70 20 Example 2 Comparative 6 FIG. 4 60 20 Example 3 Comparative 1 FIG. 4 60 20 Example 4 Comparative 1 FIG. 4 60 20 Example 5

TABLE 2-2 Proportion(%) of pore volume to total pore volume C Zeolite NOx Pore volume Pore volume supporting conver- A (0.5α B (2α Total amount sion or less) or more) (A + B) (g/L) rate (%) Example 1 4.7 6.2 10.9 120 65 Example 2 4.7 6.2 10.9 120 58 Example 3 4.7 6.2 10.9 120 60 Example 4 5.8 8.6 14.4 120 60 Example 5 4.2 5.1 9.3 120 63 Example 6 4.7 6.2 10.9 80 52 Example 7 4.7 6.2 10.9 150 56 Reference 10.8 8.9 19.7 120 50 Example 1 Comparative 4.5 4.9 9.4 120 45 Example 1 Comparative 8.2 9.4 17.6 120 48 Example 2 Comparative 10.6 10.9 21.5 120 42 Example 3 Comparative 4.7 6.2 10.9 70 45 Example 4 Comparative 4.7 6.2 10.9 160 43 Example 5

As in Examples and Reference Example, when the porosity is from about 55% to about 65%, the zeolite supporting amount is from about 80 g/L to about 150 g/L, and when the proportion of the sum of the pore volume A and the pore volume B relative to the total pore volume C is about 20% or less, the NOx conversion rate is as high as 50% or more.

Further, when the pore volume A is about 10% or less of the total pore volume C, and the pore volume B is about 10% or less of the total pore volume C, the NOx conversion rate is further increased.

(Example 5: when the sum of the pore volume A and the pore volume B is 9.3% of the total pore volume C, the NOx conversion rate is 63%.)

On the other hand, as in Comparative Examples 1 to 5, when any of the porosity, the zeolite supporting amount, and the sum of the pore volume A and the pore volume B is not included within the above ranges, the NOx conversion rate is as low as less than 50%.

In Examples, Reference Example, and Comparative Examples, a β-type zeolite ion-exchanged with an iron ion was used; however, the same results are considered to be obtained when a β-type zeolite ion-exchanged with a copper ion or a zeolite other than β-type zeolite are used.

Second Embodiment

The following description will discuss a second embodiment which is the one embodiment of the present invention.

In the present embodiment, the honeycomb structure forming the honeycomb filter includes a single honeycomb fired body. The honeycomb structure including a single honeycomb fired body is also referred to as an integral honeycomb structure.

FIG. 7A is a perspective view that schematically shows one example of the honeycomb structure forming the honeycomb filter according to the second embodiment of the present invention, and FIG. 7B is a B-B line cross-sectional view of the honeycomb structure shown in FIG. 7A.

A honeycomb structure 80 shown in FIG. 7A has a substantially round pillar shape having a first end face 84 and a second end face 85. The honeycomb structure 80 includes large volume cells 91 a each having a relatively larger cross-sectional area perpendicular to the longitudinal direction thereof (direction of the double-sided arrow “c” in FIG. 7A) than small volume cells 91 b and the small volume cells 91 b each having a relatively smaller cross-sectional area perpendicular to the longitudinal direction thereof than the large volume cells 91 a.

The large volume cells 91 a have a substantially octagonal shape in the cross section perpendicular to the longitudinal direction thereof, and the small volume cells 91 b have a substantially quadrangular shape in the cross section perpendicular to the longitudinal direction thereof.

A coat layer 82 is formed on a peripheral side face of the honeycomb structure 80.

The main component of constituent materials of the integral honeycomb structure is desirably cordierite or aluminum titanate.

The large volume cell 91 a is open at an end portion on the first end face 84 side of the honeycomb structure 80 and is sealed with a sealing material 92 a at an end portion on the second end face 85 side of the honeycomb structure 80. On the other hand, the small volume cell 91 b is open at an end portion on the second end face 85 side of the honeycomb structure 80 and is sealed with a sealing material 92 b at an end portion on the first end face 84 side of the honeycomb structure 80. A cell wall 93 interposed between the large volume cell 91 a and the small volume cell 91 b functions as a filter.

Namely, exhaust gases introduced to the large volume cell 91 a surely passes through the cell walls 93 and then flows out from the small volume cell 91 b.

The porosity, average pore diameter, and pore diameter distribution of the honeycomb structure of the present embodiment are the same as those of the cell walls of the honeycomb structure according to the first embodiment of the present invention.

The honeycomb filter of the present embodiment is obtained by allowing the cell walls of the honeycomb structure to support zeolite.

Kinds of zeolite, amount of zeolite supported on the cell walls of the honeycomb structure are the same as those in the first embodiment of the present invention.

In order to manufacture the honeycomb filter of the present embodiment, the honeycomb molded body is manufactured in the same manner as in the first embodiment of the present invention, except that the honeycomb molded body manufactured by extrusion molding is larger than and in a different outer shape from the honeycomb molded body described in the first embodiment of the present invention.

Other procedure is almost the same with the manufacturing process of the honeycomb filter according to the first embodiment of the present invention. However, since the honeycomb structure forming the honeycomb filter includes a single honeycomb fired body in the present embodiment, the bonding is not necessary. Moreover, when a honeycomb molded body in a substantially round pillar shape is manufactured, the periphery cutting is not necessary.

It is possible to manufacture a urea SCR device using the thus manufactured honeycomb filter in the same manner as in the first embodiment of the present invention.

The honeycomb filter of the present embodiment can exert the same effects (1) to (5) as those of the first embodiment of the present invention.

Other Embodiments

In manufacturing a honeycomb filter by using an aggregated honeycomb structure, in the first embodiment of the present invention, a honeycomb structure is allowed to support zeolite. Further, it is possible to allow a plurality of honeycomb fired bodies to support zeolite, and then the zeolite-supporting honeycomb fired bodies are bonded with one another by interposing the adhesive layer therebetween.

In the honeycomb filter according to the embodiments of the present invention, the shapes of the large volume cell and the small volume cell included in the honeycomb structure are not limited to the shapes explained in the foregoing embodiments.

FIG. 8A, FIG. 8B, FIG. 8C, and FIG. 8D are each a side view that schematically shows one example of the first end face of the honeycomb fired body forming the aggregated honeycomb structure according to an embodiment of the present invention.

Those figures are side views seen from the first end face side of the honeycomb fired body, namely, from the end face side where the small volume cells are sealed.

The following description will discuss the cross sectional shapes of the large volume cells and the small volume cells in other embodiments of the honeycomb structure with reference to the figures.

In a honeycomb fired body 110 shown in FIG. 8A, a cross section perpendicular to the longitudinal direction of a large volume cell 111 a has a substantially quadrangular shape in which portions corresponding to corners each have an arcuate shape.

In a honeycomb fired body 120 shown in FIG. 8B, cross sections perpendicular to the longitudinal direction of a large volume cell 121 a and a small volume cell 121 b have curved sides.

Namely, in FIG. 8B, the shape of a cross section of a cell wall 123 drawn in a solid line is a curved line.

In the shape of the cross section of the large volume cell 121 a, the cell wall 123 is convex to the outside from the center. On the other hand, in the shape of the cross section of the small volume cell 121 b, the cell wall 123 is convex to the center from the outside.

The cell wall 123 has a wave shape that rises and falls in the horizontal and perpendicular directions of a cross section of the honeycomb fired body. Mountain portions (portions that exhibits the maximum amplitude in the sine curve) of the wave shape of the adjacent cell walls 123 make their closest approach to one another, whereby the large volume cells 121 a whose cross section is dented outward and the small volume cells 121 b whose cross section is dented inward are formed. The amplitude of the wave shape may be constant or variable, but it is desirably constant.

In a honeycomb fired body 130 shown in FIG. 8C, a cross section of a large volume cell 131 a perpendicular to the longitudinal direction has a substantially pentagonal shape in which three corners have substantially right angles. A cross section of a small volume cell 131 b perpendicular to the longitudinal direction has a substantially quadrangular shape, and the substantially quadrangular shape and another substantially quadrangular shape are placed at portions of a greater quadrangle to diagonally face each other.

In a honeycomb fired body 140 shown in FIG. 8D, a cross section of a large volume cell 141 a perpendicular to the longitudinal direction and a cross section of a small volume cell 141 b perpendicular to the longitudinal direction both have a substantially quadrangular shape (substantially rectangle), and two large volume cells and two small volume cells are combined together to form an approximately square shape.

An integral honeycomb structure may have the cross-sectional shapes of the large volume cells and the small volume cells shown in FIG. 8A, FIG. 8B, FIG. 8C, and FIG. 8D.

The cross-sectional shape of the cells placed perpendicular to the longitudinal direction of the honeycomb structure is defined by the shape of the cells excluding imperfect cells (cells with a partially cut off cross section).

The shape of the honeycomb filter is not limited to a substantially round pillar shape, and may be optional pillar shapes such as a substantially cylindroid shape and a substantially polygonal pillar shape.

The cell density on a cross section perpendicular to the longitudinal direction of the honeycomb structure is not particularly limited. A desirable lower limit is about 31.0 pcs/cm² (about 200 pcs/in²) and a desirable upper limit is about 93 pcs/cm² (about 600 pcs/in²). A more desirable lower limit is about 38.8 pcs/cm² (about 250 pcs/in²) and a more desirable upper limit is about 77.5 pcs/cm² (about 500 pcs/in²).

The thickness of the cell wall of the honeycomb structure is not particularly limited and is desirably from about 0.2 mm to about 0.4 mm.

The main component of constituent materials of the honeycomb fired body forming the aggregated honeycomb structure and the integral honeycomb structure is not limited to silicon carbide, and may include other ceramic material, for example, a nitride ceramic such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; a carbide ceramic such as zirconium carbide, titanium carbide, tantalum carbide, and tungsten carbide; and oxide ceramics such as alumina, zirconia, cordierite, mullite, and aluminum titanate, and the like.

Non-oxide ceramics are desirable for use as the main component of the constituent material of the honeycomb structure among the possible components, and silicon carbide is particularly desirable. This is because silicon carbide is excellent in thermal resistance, mechanical strength, and thermal conductivity. Moreover, silicon-containing ceramics, which contains metallic silicon with the above-mentioned ceramics, and ceramics bound by silicon or silicate compounds can also be used as the constituent material of the honeycomb structure. And out of these, those (silicon-containing silicon carbide) of ceramics containing metallic silicon are preferable.

In particular, it is desirable to use a silicon-containing silicon carbide containing about 60% by weight or more of silicon carbide.

The particle diameter of the ceramic powder in the wet mixture to be used in preparing the honeycomb fired bodies forming the aggregated honeycomb structure and the integral honeycomb structure is not particularly limited, and the ceramic powder that tends not to cause the case where the size of the honeycomb fired body manufactured by the subsequent firing treatment becomes smaller than that of a degreased honeycomb molded body manufactured through the degreasing is preferable. A powder having a combination of 100 parts by weight of powder having an average particle diameter of from about 1.0 μm to about 50 μm, and from about 5 parts by weight to about 65 parts by weight of powder having an average particle diameter of from about 0.1 μm to about 1.0 μm is preferable.

As mentioned earlier, when ceramic powder having a uniform particle diameter is obtained by removing ceramic powder having particle diameters in a predetermined range through sieving of the ceramic powder, and then the ceramic powder having a uniform particle diameter is used, the pore diameter distribution may be adjusted in a predetermined range.

The organic binder in the wet mixture to be used in preparing the honeycomb fired bodies forming the aggregated honeycomb structure and the integral honeycomb structure is not particularly limited, and examples thereof include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene glycol, and the like. Methylcellulose is preferable among the above examples. In general, the blending amount of the organic binder is desirably from about 1 part by weight to about 10 parts by weight with respect to 100 parts by weight of the ceramic powder.

The plasticizer to be contained in the wet mixture is not particularly limited, and examples thereof include glycerin or the like.

Also, the lubricant to be contained in the wet mixture is not limited, and examples thereof include polyoxy alkylene-based compounds such as polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, and the like.

Specific examples of the lubricant include polyoxyethylene monobutyl ether, polyoxypropylene monobutyl ether, and the like.

Here, the plasticizer and the lubricant are not necessarily contained in the wet mixture depending on cases.

Upon preparing the wet mixture, a dispersant solution may be used, and examples of the dispersant solution include water, an organic solvent such as benzene, alcohol such as methanol, and the like.

Moreover, a molding auxiliary may be added to the wet mixture.

The molding auxiliary is not particularly limited, and examples thereof include ethylene glycol, dextrin, fatty acid, fatty acid soap, polyalcohol, and the like.

Furthermore, the pore-forming agent to be added in the wet mixture is not limited to hollow acrylic particles and may be balloons that are fine hollow spheres including an oxide-based ceramic, graphite, and the like.

With respect to the balloons, not particularly limited, for example, alumina balloons, glass micro-balloons, shirasu balloons, fly ash balloons (FA balloons), mullite balloons and the like may be used. Out of these, alumina balloons are more desirably used.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

1. A honeycomb filter comprising: a honeycomb structure having cell walls extending along a longitudinal direction of the honeycomb structure to define cells between the cell walls, each of the cells being sealed at either end of the cells; and a zeolite supported on the cell walls of the honeycomb structure, wherein an amount of the zeolite supported on the cell walls is from about 80 g/L to about 150 g/L, the cells include a large volume cell and a small volume cell, a porosity of the cell walls of the honeycomb structure is from about 55% to about 65%, an average pore diameter of the cell walls of the honeycomb structure is from about 15 μm to about 25 μm, and a proportion of a sum of a pore volume A and a pore volume B relative to a total pore volume C is about 20% or less in a pore diameter distribution of the cell walls of the honeycomb structure, the pore volume A indicating a pore volume of pores having a pore diameter of one half or less of the average pore diameter, the pore volume B indicating a pore volume of pores having a pore diameter of twice or more the average pore diameter.
 2. The honeycomb filter according to claim 1, wherein the pore volume A is about 10% or less of the total pore volume C, and the pore volume B is about 10% or less of the total pore volume C.
 3. The honeycomb filter according to claim 1, wherein a cross section of the large volume cell perpendicular to the longitudinal direction has a substantially octagonal shape, and a cross section of the small volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape.
 4. The honeycomb filter according to claim 1, wherein a cross section of the large volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape, and a cross section of the small volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape.
 5. The honeycomb filter according to claim 1, wherein the zeolite comprises at least one of β-type zeolite, Y-type zeolite, ferrierite, ZSM-5 type zeolite, mordenite, faujasite, A-type zeolite, L-type zeolite, SAPO, and MeAPO.
 6. The honeycomb filter according to claim 5, wherein the zeolite comprises at least one of a β-type zeolite, a ZSM-5 type zeolite, and a SAPO.
 7. The honeycomb filter according to claim 6, wherein the SAPO comprises at least one of SAPO-5, SAPO-11, and SAPO-34.
 8. The honeycomb filter according to claim 5, wherein the MeAPO comprises MeAPO-34.
 9. The honeycomb filter according to claim 1, wherein the zeolite is ion-exchanged with metal ions.
 10. The honeycomb filter according to claim 9, wherein the metal ions comprise at least one of a copper ion, an iron ion, a nickel ion, a zinc ion, a manganese ion, a cobalt ion, a silver ion, and a vanadium ion.
 11. The honeycomb filter according to claim 10, wherein the zeolite is ion-exchanged with at least one of a copper ion and an iron ion.
 12. The honeycomb filter according to claim 1, wherein the honeycomb structure comprises a plurality of honeycomb fired bodies combined with one another with an adhesive layer interposed between the plurality of honeycomb fired bodies.
 13. The honeycomb filter according to claim 12, wherein a main component of constituent materials of the honeycomb structure comprises at least one of silicon carbide and silicon carbide containing silicon.
 14. The honeycomb filter according to claim 1, further comprising: a coat layer formed on a periphery of the honeycomb structure.
 15. The honeycomb filter according to claim 1, wherein an area ratio of a cross-sectional area of the large volume cell perpendicular to the longitudinal direction relative to a cross-sectional area of the small volume cell perpendicular to the longitudinal direction is from about 1.4 to about 2.4.
 16. The honeycomb filter according to claim 1, wherein the amount of the zeolite supported on the cell wall is from about 120 g/L to about 150 g/L.
 17. The honeycomb filter according to claim 1, wherein the honeycomb structure comprises a single honeycomb fired body.
 18. The honeycomb filter according to claim 17, wherein a main component of constituent materials of the honeycomb structure comprises at least one of cordierite and aluminum titanate.
 19. The honeycomb filter according to claim 1, wherein a cross section perpendicular to a longitudinal direction of the large volume cell has a substantially quadrangular shape in which portions corresponding to corners each have an arcuate shape, and a cross section perpendicular to a longitudinal direction of the small volume cell has a substantially quadrangular shape.
 20. The honeycomb filter according to claim 1, wherein cross sections perpendicular to a longitudinal direction of the large volume cell and the small volume cell have curved sides.
 21. The honeycomb filter according to claim 1, wherein a cross section of the large volume cell perpendicular to the longitudinal direction has a substantially pentagonal shape in which three corners have substantially right angles, a cross section of the small volume cell perpendicular to the longitudinal direction has a substantially quadrangular shape, and the substantially quadrangular shape and another substantially quadrangular shape are placed at portions of a greater quadrangle to diagonally face each other.
 22. The honeycomb filter according to claim 1, wherein a cross section of the large volume cell perpendicular to the longitudinal direction and a cross section of the small volume cell perpendicular to the longitudinal direction both have a substantially quadrangular shape, two large volume cells and two small volume cells being combined together to form an approximately square shape.
 23. The honeycomb filter according to claim 1, wherein the honeycomb filter is used as a urea SCR device. 